The present invention relates generally to melter kettles that are designed and used to melt thermoplastic materials that are applied to pavements such as roadways, airport runways, parking lots, bicycle paths and other surfaces requiring pavement markings. More particularly the present invention is directed to systems and methods to improve the efficiency of melter kettles.
Thermoplastic materials are the product of choice for many types of pavement marking operations. However, unlike most types of marking materials thermoplastic materials must be heated to relatively high temperatures that can reach to about 420° F. to be melted and fluid enough to be applied.
Early types of thermoplastic application equipment applied the thermoplastic at slow rates. Therefore, the long melting times it took to melt thermoplastic materials in melter kettles were not a problem. Melter kettles could keep up with the slow output of application equipment.
Eventually improvements in the designs of melter kettles achieved reductions of melting times. However, over time application equipment was improved to the point at which thermoplastic material could be applied at much faster rates than the improved melter kettles could keep up with melting the thermoplastic material. The present invention increases the efficiency of melting thermoplastic in melter kettles that can be mounted on either thermoplastic application trucks, nurse trucks, trailers or the like.
For some time heat domes, also called heat risers or heat tubes, have been installed in melter kettles. The dome structure is formed by a tube of variable diameter that is attached to a hole in the base of the melter kettle where the OD of the dome base matches the ID of the hole in the base of the melter kettle. The top of the dome is closed by a metal disc. The dome reduces the heating surface area of the base. However, the dome provides additional circumference surface area that compensates for the loss of the heating area in a melter kettle with no dome and compensates for the lost surface area of the base within a few inches of dome height. From this point the dome adds melting (heat transfer) surface area to the melter kettle with a dome as compared to a melter kettle without a dome thereby increasing the overall heating surface area in the melter kettle that acts on the thermoplastic material in the melter kettle. This reduces the ratio of the thermoplastic material to melting (heat transfer) surface area of the melter kettle which improves heating efficiency.
Additionally, heating thermoplastic material in a melter kettle from the middle of the melter kettle in an outwardly direction is more efficient than heat transfer from the outside of the melter kettle in an inward direction. Heat domes have reduced melting times in melter kettles. However, heated air in the dome cools as heat transfers through the dome wall and into the thermoplastic melter kettle. Melting times are reduced with the use of domes but still need improvement.
A recent improvement in melter kettle efficiency has been developed by the present inventor and is disclosed in U.S. non-provisional application Ser. No. 15/424,451 entitled “HEAT DOME TEMPERATURE REGULATING SYSTEM,” filed Feb. 3, 2017. In this copending application a heat dome chimney stack tube is attached to the top center of a heat dome about which an agitator drive shaft tube rotates. Hot combustion gasses travel from the heat dome up the center of the heat dome chimney tube stack and vent into a top tube drive shaft heat chamber that has a drive shaft tube relief vents through which combustion gasses can be regulated by providing a rotational vent relief collar about the top tube driveshaft heat chamber. This system exhausts combustion gasses from the dome that has been heat depleted thereby allowing a continual flow of hot combustion gasses to maximize/optimize efficient temperature in the dome such that the maximum amount of heat is transferred through the dome and chimney stack surface areas into the thermoplastic material in the melter kettle. In this system the heat dome chimney stack tube and rotational drive shaft become heating surfaces through the centerline of the melter kettle. This system improves the rate of thermoplastic melting.
Another recent improvement in melter kettle efficiency developed by the present inventor is disclosed in U.S. non-provisional application Ser. No. 15/424,461, entitled THERMOPLASTIC KETTLE MATERIAL CIRCULATION SYSTEM, filed Feb. 3, 2017. In this improvement a single vertical material transfer tube is affixed to the side of the thermoplastic melter kettle either directly to the melter kettle side wall or outer insulation skin. The tube is attached to ports at the bottom and top of the melter kettle and an auger rotated by a direct drive motor within the vertical transfer tube moves molten thermoplastic material from the bottom of the melter kettle to the top. When the vertical material transfer tube is connected directly to the melter kettle wall the bottom interface is within the heat chamber's outer wall.
When the vertical material transfer tube is affixed to the outer insulation skin there is an extended heat chamber surrounding the vertical material transfer tube. A port larger in diameter than the lower material transfer tube allows heat from the combustion chamber to contact the vertical material transfer tube.
Another recent improvement in melter kettle efficiency developed by the present inventor is disclosed in U.S. non-provisional patent application Ser. No. 15/424,455, entitled THERMOPLASTIC KETTLE OIL BATH AUXILIARY HEAT EXCHANGER SYSTEM, filed Feb. 3, 2017. This invention combines an odd number of interconnected vertical tubes within an oil bath through which heated heat transfer oil flows. The function of the system is to increase the temperature of molten thermoplastic material moving through the circuit of interconnected heat transfer tubes by action of an independent high BTU output furnace that heats circulated heat transfer oil that circulates around the interconnected tubes. Molten thermoplastic material enters the base of the first tube through a melter kettle bottom material flow port and the tube bottom material flow port both of which are isolated from the oil bath. The molten thermoplastic material reenters the melter kettle at the top center through the top flow tube that connects to the top of the discharge tube that is above the level of the melter kettle top and is isolated from the oil bath. Each tube contains an auger.
The augers are interconnected by a gear train. A single hydraulic motor attached to any auger drives each gear and auger in a counter rotational direction. This circulates the molten thermoplastic material from the bottom of the melter kettle where it is hottest through the melter kettle bottom material flow port into the bottom of the first tube then up and down the plurality of tubes. The material flows up the last tube and through a tube top port which is isolated from the oil bath and through the top material flow tube located at a level above the top of the melter kettle fill line. The molten thermoplastic material is deposited near the top center of the melter kettle where it heats and displaces downward the thermoplastic material at the surface of the melter kettle. The heat transfer enters the oil bath tub adjacent the thermoplastic material discharge port where both the oil and thermoplastic material are at their hottest temperature and is directed through and leaves the system adjacent the thermoplastic material inlet port where it is heat depleted. When the system is disengaged and circulation ceases the hydraulic motors are run in a reverse direction to purge as much thermoplastic material from all tubes except for the inlet tube. This will leave solid material in only the first tube so that when the system is restarted it will take less heat and hydraulic energy to engage the system and begin moving molten thermoplastic material.
There is a limit to the various available energy outputs of mobile equipment systems that can be incorporated in thermoplastic equipment such as heat, electrical, engine, hydraulic, air and other systems. Some serious drawbacks to thermoplastic oil bath auxiliary heat exchanger systems are that they require a separate high BTU boiler system, separate hot oil circuits as well as oil expansion chambers designed with exotic heat transfer oils some of which require inert gas blanket interfacing. The high output boilers required need more space than is available on most thermoplastic application trucks. Motors to run the hydraulics and oil circulation systems are subject to space limitations. Weight is also a serious consideration. For each pound that the system weighs the load carrying capacity is reduced by a similar amount. Costs are high for all of the system components.
Another recent improvement in melter kettle efficiency that has been developed by the present inventor is disclosed in U.S. non-provisional patent application Ser. No. 15/424,467, entitled THERMOPLASTIC KETTLE AUXILIARY HEAT EXCHANGER SYSTEM filed Feb. 3, 2017. This system is a design that allows a plurality of interconnected tubes to be used like those in copending non-provisional application Ser. No. 15/424,455 where the plurality of tubes are within the heat chamber and not an oil bath. This eliminates the need for additional furnaces, pumps, hydraulic systems and an oil bath chamber that are required in the oil bath invention in copending non-provisional application Ser. No. 15/424,455. The interconnected plurality of tubes with auger assemblies is connected directly to the inner wall of the heat chamber. The inlet is at the bottom of the first tube's intake port and the outlet is at the top of the top of the last tube's outlet port above the fill line of the melter kettle.
There is a critical difference in both design and function of the oil bath auxiliary heat exchanger and the heat chamber auxiliary heat exchanger. In an oil bath system the thermoplastic material can never go above the temperature of the heat transfer oil. The heat transfer oil's highest operation temperature cannot exceed the baking/degradation temperature of the thermoplastic material. Therefore the oil bath system is a failsafe system with respect to the temperature at which thermoplastic material is heated. In non-oil bath heating systems the heat chamber can exceed the baking/degradation temperature of the thermoplastic material. To prevent baking/degradation in the heat chamber system special procedures must be followed. The thermoplastic material must be constantly moving through the system during operation. At shut down the thermoplastic material must continue circulating until the melter kettle and tube walls drop below a safe temperature. It may be necessary to add ambient temperature material to the melter kettle to draw down the heat on the melter kettle and tube walls. The direction of flow in the tubes must never be reversed until a safe temperature is reached or the augers may be frozen in place.
The present invention provides an improvement for melter kettles used for melting thermoplastic pavement marking material wherein the melter kettle is provided with a combustion chamber the improvement comprising an auxiliary tube bundle multi-pass tube bundle exchanger (also referred to herein as a multi-pass tube bundle thermoplastic pavement marking material heat exchanger) wherein molten thermoplastic pavement marking material enters a bottom inlet in the auxiliary multi-pass tube bundle heat exchanger from a corresponding bottom outlet of the melter kettle bottom and through a connecting transfer tube. The auxiliary multi-pass tube bundle heat exchanger has an odd numbered multi-pass assembly that allows the molten thermoplastic material to circulate such that it exits the last vertical section through a top outlet of the heat exchanger to a corresponding top inlet of the melter kettle through a top connecting transfer tube. Movement of the molten thermoplastic material from the melter kettle, through the auxiliary multi-pass tube bundle heat exchanger and back into the melter kettle is achieved by means of any type of pump suitable for the purpose.
A hot oil circulation system coupled to the auxiliary multi-pass tube bundle heat exchanger heats the thermoplastic material flowing through the auxiliary multi-pass tube bundle heat exchanger by heat transfer across the tube walls and into the thermoplastic material. The hot oil circulation system includes a high BTU output oil furnace with temperature controls, a pump designed to circulate the oil through the circuit and flow lines. The heated oil is pumped from the furnace through the flow lines connected to the oil inlet port of the auxiliary multi-pass tube bundle heat exchanger located at the top of the auxiliary multi-pass tube bundle heat exchanger just below the top of the tube bundle terminus wherein it circulates downward and exits the oil outlet port just above the bottom bundle terminus. The heat depleted oil returns through the return flow lines back to the furnace where it is reheated and returned to the hot oil pump.